This invention relates to sensors for use in electric arc welding processes. More particularly, the present invention relates to infrared sensors capable of directly viewing the arc region and surrounding workpiece areas.
In various electric arc welding processes, it is desirable to be able to determine a number of weld parameters during the welding operation itself. This is particularly important for automated welding processes. At present, automated welding processes have been essentially limited to spot welding processes. However, continued development of automated welding processes, particularly those processes carried out by general purpose manufacturing robots, require greater amounts of continuous information relevant to the quality of the weld that is being made. This information must, of necessity, be furnished on a real time basis, as the weld is being made. In short, there is a significant need for feedback control mechanisms based on such information to control certain parameters of the welding process. These parameters include arc current, arc voltage, electrode distance to the workpiece, lateral torch positioning and rate of torch movement along the length of the weld. Another significant variable that is extremely useful in determining the ultimate strength and quality of a welded joint is the penetration depth of the weld. This variable describes the depth that the weld fuses into the root of the joint. Because a stress joint is likely to fail at points where incomplete penetration has occurred, the continuous control of weld penetration is a significantly desirable objective in any automated welding process. The most influential factor determining weld depth penetration is the heat input per unit length of the weld pass. Increasing the arc current or decreasing the travel speed can, for example, result in greater penetration. However, other factors beyond the direct control of the welder, such as joint design and fit-up, also have important influences on weld depth penetration. Thus, it becomes highly desirable that automatic welders and processes employ some means of detecting penetration and of adjusting the heat supplied to the weld joint, to ensure proper quality for the finished weld.
Several different approaches have been taken in the past to determine weld depth penetration and quality. The most direct indication of heat input rate during a welding operation is the temperature of the workpiece at some fixed point relative to the arc. Thus, conventional temperature sensors have been employed to measure changes in such temperatures and, through appropriate feedback controls, changes in controllable parameters have been made to occur. These parameters have included arc current, torch travel speed and arc length. In the Sciaky weld penetration control, infrared sensors continuously monitor the heated zone on the underside of the joint that is being welded. This process possesses the significant disadvantage that uninhibited access to the underside of the workpiece is required. Additionally, critical alignment requirements necessitate the synchronization of the sensor and its movements with the welding torch movements. Thus, in the Sciaky control, there is no direct observation of the weld pool itself.
Other work in this field has been reported in a progress report titled "Improvement of Reliability of Welding by In-process Sensing and Control(Development of Smart Welding Machines for Girth Welding of Pipes)" submitted to the Department of Energy in June, 1981 by Jose Converti, et al. This report describes initial experiments conducted using contact sensors (thermocouples) to probe the temperature distribution near the weld puddle and seam. Attempts to use near infrared photodiodes, described therein, for remote temperature sensing were not successful due to significant optical interference from plasma radiation reflected from the metal surface. In particular, Converti et al. propose using a simple optical filter to reduce the radiation from the plasma arc through the use of materials similar to conventional welders' goggles.
A significantly different technique, based on contact thermometry has been developed by NASA for aluminum welding. A constantan wire, making a sliding contact with the workpiece, forms one leg of a thermocouple circuit. This contact occurs near the weld on the torch side of the joint. Such a system appears to function more successfully as a penetration control device on thin (0.125") aluminum workpieces rather than on thicker pieces where complete penetration is necessary.
In short, prior workers in the field of automated arc welding processes have yet to provide high resolution data and detailed information about the weld pool and the area immediately surrounding the arc. However, the instant inventor has discovered significant information concerning the spectral distribution of the infrared radiation from the arc itself, thus enabling the construction of an arc welding sensor providing hither-to unavailable information concerning weld quality, on a continuous, real-time basis, from the critically important region near to the arc and weld pool.